Traditionally “optical coherence tomography” (OCT) has been used to understand information of the inside of an object, or specifically the differential structure of refractive index distribution, in a nondestructive manner at high resolution.
OCT is a nondestructive tomography measurement technology used in the medical field, etc. (refer to Patent Literature 1). OCT uses light as probe beam (measurement light), which provides the advantage of being able to measure refractive index distribution, spectral information and polarization information (birefringence distribution) of a measuring target, among others.
A basic OCT 53 is based on Michelson's interferometer and its operating principle is explained using FIG. 7. Light emitted from a light source 44 is paralleled by a collimator lens 45, and then split into reference light and object light via a beam splitter 46. Object light is condensed into a measuring target 48 by an object lens 47 in the object arm, where it is scattered and reflected and then returned to the object lens 47 and beam splitter 46.
On the other hand, reference light passes through an object lens 49 in the reference arm, after which it is reflected by a reference mirror 50 and returned to the beam splitter 46 via the object lens 49. Thus returned to the beam splitter 46, this reference light enters a condensing lens 51, together with the object light, and both are condensed into an optical detector 52 (photodiode, etc.).
The light source 44 of the OCT utilizes light of temporally low coherence (light that makes it extremely unlikely for lights emitted from the light source at different timings to interfere with each other). With Michelson's interferometer, which uses temporally low coherence light as the light source, interference signals manifest only when the distance of the reference arm is roughly equivalent to the distance of the object arm. As a result, an interference signal relative to optical path length difference (interferogram) is obtained when the interference signal intensity is measured with the optical detector 52 by changing the optical path length difference (τ) of the reference arm and object arm.
The shape of this interferogram represents the reflectance distribution of the measuring target 48 in the depth direction, where the structure of the measuring target 48 in the depth direction can be obtained by one-dimensional scanning in the axial direction. In other words, the OCT 53 can measure the structure of the measuring target 48 in the depth direction by means of optical path length scanning.
Two-dimensional scanning, comprising mechanical scanning in the lateral direction in addition to the above scanning in the axial direction, can obtain a two-dimensional section image of the measuring target. A scanning apparatus that performs the above scanning in the lateral direction may be structured to directly move the measuring target, structured to shift the object lens with the measuring target fixed, or structured to rotate the angle of the galvanometer mirror placed near the pupil surface of the object lens with the measuring target and object lens fixed, among others.
Advanced versions of the aforementioned basic OCT include swept source OCT (“SS-OCT”) where the wavelength of the light source is scanned to obtain spectral interference signals, and spectral domain OCT (“SD-OCT”) where a spectroscope is used to obtain spectral interference signal lights. Fourier domain OCT (“FD-OCT”; refer to Patent Literature 2) and polarization-sensitive OCT (“PS-OCT”; refer to patent Literature 3) are examples of the latter.
In SS-OCT, a high-speed wavelength scanning laser is used to change the wavelength of the light source, and the light-source scanning signals obtained synchronously with spectral signals are used to rearrange interference signals, to which signal processing is applied to obtain a three-dimensional optical tomography image. SS-OCT can also use a monochrometer as a means for changing the wavelength of the light source.
In FD-OCT, the wavelength spectrum of the reflected light from the measuring target is obtained with a spectrometer and the resulting spectral intensity distribution is Fourier-transformed to extract signals in the actual space (OCT signal space). This FD-OCT does not require scanning in the depth direction, and the section structure of the measuring target can be measured only by scanning in the x-axis direction.
PS-OCT is similar to FD-OCT in that the wavelength spectrum of the reflected light from the measuring target is obtained with a spectrometer, where the difference is that with PS-OCT, incident light and reference light are passed through a ½ wave plate and ¼ wave plate, etc., respectively, for horizontal linear polarization, vertical linear polarization, 45° linear polarization or circular polarization, and reflected light and reference light from the measuring target are superimposed and passed through a ½ wave plate, ¼ wave plate, etc., to cause only the horizontally polarized light component to enter the spectrometer to cause interference, for example, thereby extracting and Fourier-transforming only the component of object light having a specific polarization condition. This PS-OCT does not require scanning in the depth direction, either.
Furthermore, technologies using Doppler optical coherence tomography (“Doppler OCT”), such as technology to measure the blood flow distribution of retina, technology to form a transverse blood flood image of retina, and technology to three-dimensionally observe the capillary structure of retina, are known, among others. In Doppler OCT, the blood flow rate, etc., is obtained by utilizing the fact that the amount of temporal change in phase (change in frequency) obtained by Fourier transformation of spectral interference information corresponds to the moving speed of the target as the Doppler signal, where SS-OCT, FD-OCT, etc., can be applied (refer to Patent Literatures 4, 5 and Non-patent Literatures 1, 2).
The inventors named under the present application for patent have also proposed a quantitative measurement apparatus for eye fundus blood flow volume, whereby the structure of blood vessels of the retina is extracted by means of Doppler OCT angiography to allow for quantification of blood flow volume in the blood vessels of the retina (refer to Japanese Patent Application No. 2008-8465).